Part of "How It Gets Built" - Buildermuse's series on the machinery behind modern construction.
Ascent, a 25-story apartment tower in downtown Milwaukee, stands 284 feet tall and is now recognized as the tallest mass timber building in the world, ahead of Norway's 18-story Mjøstårnet, according to Wisconsin Public Radio's November 23, 2023 report. The building's first six floors and its elevator shafts are concrete; the floors above use glue-laminated columns and beams with timber floor panels shipped from Austria. Residents started moving into its 259 apartments on July 15, 2022 (FOX6 Milwaukee, July 14, 2022).
Ascent is the headline, but the volume is in smaller buildings. WoodWorks, the wood industry's nonprofit technical support group, reports that 2,833 multifamily, commercial or institutional mass timber projects were in progress or built in the U.S. as of June 2026, up from 2,115 in March 2024 (WoodWorks, Mapping Mass Timber). Most of those projects are well under 12 stories. If you bid commercial or multifamily work, the odds that a mass timber job shows up in your market keep going up.
This article covers what the materials are, what the building code allows, how the frame goes up, and which trades have to change how they work.
What CLT and Glulam Actually Are
Mass timber comes down to two kinds of product.
Cross-laminated timber (CLT) is the flat product: layers of lumber glued together, with each layer running at 90 degrees to the one below. The cross-lamination gives the panel strength in both directions, so it can work as a floor, roof or wall panel. Glue-laminated timber (glulam) is the linear product: laminations glued with the grain running the same direction to make beams and columns. Most tall timber buildings combine the two, with glulam posts and beams carrying CLT floor panels, and a concrete or steel core for stairs and elevators.
The 2021 International Building Code also recognizes nail-laminated timber (NLT), dowel-laminated timber (DLT) and structural composite lumber panels as mass timber, but only when they meet the code's minimum size and detailing rules, according to a WoodWorks and American Wood Council technical paper on tall wood in the 2021 IBC (revised February 2022). That distinction matters for estimators: a panel product used in a Type III or Type V building does not automatically have to meet Type IV dimension rules.
Where the panels come from
Ascent's timber came from Austria by container ship and then by rail through Chicago, according to the developer, New Land Enterprises, as quoted by WPR. North American capacity has grown since then. One example: Mercer International bought the former Katerra CLT plant in Spokane, Washington, for $50 million in August 2021, and Mercer put the plant's capacity at about 140,000 cubic meters a year (Timber-Online, August 5, 2021).
For a contractor, the practical point is that panels are made to order from shop drawings. Penetrations, notches and connection pockets are usually cut at the plant on CNC equipment, so the design and MEP coordination have to be finished before fabrication. A late or wrong panel package can stop the frame.
The Code: Type IV-A, IV-B and IV-C
The International Code Council approved the tall wood proposals in January 2019, and the 2021 IBC added three construction types based on the old heavy timber type (now called Type IV-HT). The limits below are for sprinklered buildings, from IBC Tables 504.3 and 504.4 as summarized in the WoodWorks/AWC paper:
| Construction type | Max height | Max stories, Business (B) | Max stories, Residential (R-2) | Max stories, Assembly (A-2, A-3, A-4) |
|---|---|---|---|---|
| IV-A | 270 ft | 18 | 18 | 18 |
| IV-B | 180 ft | 12 | 12 | 12 |
| IV-C | 85 ft | 9 | 8 | 6 |
| IV-HT (heavy timber) | 85 ft | 6 | 5 | 4 |
Source: WoodWorks / American Wood Council, Tall Wood Buildings in the 2021 IBC.
The three new types differ mainly in how much of the timber has to be covered with noncombustible protection such as gypsum board:
- Type IV-A: all mass timber requires noncombustible protection. Base fire-resistance ratings match Type I-A, including 3 hours for the primary structural frame.
- Type IV-B: most mass timber requires protection, with limited exposed areas. Fire-resistance ratings match Type I-B, including 2 hours for the primary frame.
- Type IV-C: most mass timber can be exposed, but the primary frame still needs a 2-hour rating in addition to heavy timber minimum sizes.
The code set the noncombustible protection at no less than two-thirds of the required fire-resistance rating for the timber element.
The 2024 IBC loosened the Type IV-B exposure limit. Under proposal G147-21, exposed mass timber ceilings in a dwelling unit or fire area can equal up to 100% of the floor area, up from 20% in the 2021 code, with exposed walls and attached columns capped at 40% of floor area (Code Red Consultants, November 14, 2022). That change cuts gypsum board and finishing labor on IV-B projects in jurisdictions that adopt the 2024 code.
Which states allow it
WoodWorks' code status page, dated September 2026, lists 42 states where the tall mass timber provisions have been adopted statewide or by major jurisdictions within the state (WoodWorks expert tip, status as of September 2026). In 35 of those, adoption is statewide on the 2021 or 2024 IBC. In seven (Arizona, Colorado, Delaware, Kansas, Missouri, New York and Texas), adoption varies by jurisdiction. WoodWorks notes that some cities adopted the provisions ahead of their states, so check the specific jurisdiction before you price a project on a Type IV-A, B or C basis.
Ascent itself was permitted before the tall wood provisions were widely in place in Wisconsin, through a project-specific approval. New projects in adopting jurisdictions can follow the prescriptive code path instead.
The Fire Question
Heavy timber does not behave like light-frame wood in a fire. When a large timber member burns, its surface chars, and the char layer insulates the wood behind it, which keeps carrying load. Engineers size exposed members with extra sacrificial wood to cover the required rating.
The tall wood code changes were backed by full-scale testing. The ICC Tall Wood Building committee designed, and the American Wood Council and the USDA Forest Products Laboratory managed, five compartment fire tests in a two-story mass timber structure at the ATF Fire Research Laboratory. In the first three tests, sprinklers were not activated and the fires were left to burn for at least three hours. According to the WoodWorks/AWC paper, in each case the fire stayed in the unit of origin, there was no structural failure, and exposed CLT self-extinguished; the fires decayed to hot spots and embers less than two hours in. In the last two tests, with almost all CLT exposed, sprinklers at half the NFPA 13 minimum density controlled the fire.
The same paper cites earlier ASTM E119 assembly tests in which CLT walls and floors reached up to a 3-hour fire-resistance rating.
There is one rule that lands directly on the construction schedule. The International Fire Code (Section 3308.4, added for the 2021 cycle) requires that, once a mass timber building under construction goes past six stories, noncombustible protection be installed on floors more than four levels below the active timber work, unless the fire code official approves otherwise. Standpipes and water supply also have to keep pace. That means the drywall or protection crew has to follow the frame up the building rather than starting at the end.
How the Frame Goes Up
The big difference from a cast-in-place concrete frame is that most of the work happens in the factory. On site, the work is mostly crane picks, connections and sealing.
A typical sequence on a tower like Ascent:
- Concrete podium and cores are built first, conventionally.
- Glulam columns are set and connected, usually with steel hardware and screws.
- Glulam beams are hung between columns.
- CLT floor panels are lifted directly from the delivery truck where possible, set on the beams and screwed down.
- Topping slab, acoustic layers or floor protection go down, and the next level starts.
The crew is small. WPR's report quotes the developer as saying about 12 people assembled Ascent's frame, roughly a quarter of the workforce on a normal site. That is the developer's figure, not an independent study, but it matches the general pattern: once the panels are on site, the crew is mostly a crane operator, riggers, a signal person and two or three carpenter or ironworker setting teams.
The best-documented speed example is Brock Commons Tallwood House at the University of British Columbia in Vancouver, an 18-story student residence with 17 stories of CLT floors over a concrete podium. Its mass wood structure and facade were completed less than 70 days after the prefabricated components arrived on site, and the project was expected to finish about four months faster than a typical project, according to Canadian Consulting Engineer, September 20, 2016.
Where the schedule savings come from
The schedule case for mass timber rests on three things:
- No formwork, shoring, rebar or cure time on the timber floors, so follow-on trades can start sooner.
- Fewer workers on the deck during the frame.
- A lighter structure. Timber weighs much less than concrete per floor, which can reduce foundation sizes, especially on poor soils. Whether that saving is real on your job depends on the geotechnical report and the structural design, so price it from the engineer's numbers, not from a rule of thumb.
The offsets are real too. The timber package has to be bought and detailed months earlier than concrete, and design coordination has to be finished before fabrication. Cost comparisons between timber and concrete vary widely by market, building type and structural grid, so budget each project from supplier quotes and your own general conditions rather than a published "premium" figure.
What Changes On Site
Moisture is the main risk
Wood has to stay dry during construction. Mass timber jobs usually run a written moisture management plan covering end-grain sealing, covered storage, prompt removal of standing water from decks and regular moisture readings. Water staining on exposed ceilings is expensive to fix and very visible to the owner. Your contract and schedule should say who owns temporary weather protection and who pays if panels get wet.
Trades that adapt
Carpenters and ironworkers do most of the setting. The work rewards layout accuracy, rigging skill and the ability to read fabrication drawings. For context on the labor pool, the BLS May 2025 Occupational Employment and Wage Statistics put the national median wage for carpenters at $29.12 an hour, structural iron and steel workers at $30.18, and crane and tower operators at $32.73 (BLS OEWS, May 2025 national estimates).
MEP trades feel the biggest change. Because penetrations are cut at the plant, sleeves, routes and openings have to be modeled and approved months earlier than plumbers, electricians and sheet metal shops are used to. Firms that already work from BIM and prefabricate do well. Firms that route everything in the field struggle.
Drywall and fire protection crews have a bigger role on Type IV-A and IV-B jobs because of the noncombustible protection requirements, and the IFC sequencing rule above means they work much earlier in the schedule than on a concrete job.
Cranes
A mass timber frame lives and dies by the hook. Panel picks are large but light, so crane selection is usually driven by radius and pick count rather than weight. Mid-rise timber projects often use a self-erecting or small tower crane. For how tower cranes are erected, climbed and inspected, see our teardown of how a tower crane climbs.
Insurance and lenders
Builder's risk underwriting for mass timber is still developing, and water damage is usually a bigger part of the conversation than fire. Expect carriers and lenders to ask for the fire engineering package, the moisture management plan and the protection sequence. Start those conversations early, because a first mass timber project in a market tends to take longer to insure and finance.
For broader context on which commercial building types are getting built, see our commercial construction spending data, and check the live federal bids board for public solicitations in your area.
Frequently Asked Questions
How tall can a mass timber building be under the IBC?
Under the 2021 and 2024 IBC, sprinklered Type IV-A buildings can reach 270 feet and 18 stories for business and residential occupancies. Type IV-B allows 180 feet and 12 stories. Type IV-C allows 85 feet, with 9 stories for business, 8 for R-2 residential and 6 for assembly occupancies. Taller buildings such as Ascent have been approved through project-specific processes.
How many states have adopted the tall mass timber code provisions?
WoodWorks' September 2026 status page lists 42 states that have adopted the provisions statewide or in major jurisdictions. Seven of those are marked "varies by jurisdiction." Always confirm with the local building department.
Is mass timber safe in a fire?
The code changes were based on fire testing that included five full-scale compartment tests at the ATF Fire Research Laboratory. With no sprinklers, the fires stayed in the unit of origin with no structural failure. ASTM E119 tests have shown CLT assemblies reaching up to 3-hour ratings. Type IV-A also requires noncombustible protection on all mass timber.
How fast does a mass timber frame go up?
It depends on the project, but the best-documented case is Brock Commons in Vancouver, where the mass wood structure and facade were completed in less than 70 days after components arrived on site. Ascent's developer said about 12 people assembled that building's frame.
What changed in the 2024 IBC for mass timber?
The 2024 IBC allows exposed mass timber ceilings in Type IV-B buildings up to 100% of the floor area of a dwelling unit or fire area, up from 20% in the 2021 IBC. Exposed walls and attached columns are limited to 40% of the floor area.
What is the biggest field risk on a mass timber job?
Moisture. Panels have to be kept dry from delivery through enclosure, which is why most projects use a written moisture management plan with covered storage, deck water removal and regular moisture readings.
Your Action Item for This Week
Find out which IBC edition your city and state have adopted and whether it includes the Type IV-A, B and C provisions. Then pull a recent mid-rise bid and ask one CLT or glulam supplier for a budget price on the frame. Compare it against your concrete or steel number, using your own general conditions and a realistic schedule for both options. That exercise will tell you more about mass timber in your market than any national average.
More from the How It Gets Built desk
Edited by Ray Kowalski, Methods & Machines desk. New teardowns weekly.
